The well-known and widely used Hall-Heroult process for the production of aluminum from aluminum oxide has long been a matter of interest if not concern due to the spent potlining which is produced in substantial quantities as a by-product of the process. The cathodic potlining is considered spent when it has either become cracked or so contaminated with cryolite, fluorspar, aluminum fluoride, alumina and other salts and contaminants from the molten bath that it no longer functions efficiently. With a typical volume of 40-60 tons of potlining per cell and a typical potlining life of 3-5 years, the amount of spent potlining generated in the industry is on the order of hundreds of metric tons per year.
Spent potlining presents both a resource in view of its recovery potential and a disposal problem in view of its toxic materials content. The recovery potential resides in both chemical and energy values and is subject to fluctuations in such factors as price, market demand, and other economic considerations. The disposal problem on the other hand is subject to continually increasing regulatory scrutiny and will always be a matter of concern. Primary considerations are the presence of toxic contaminants such as cyanides and fluorides which are susceptible to leaching into soil and groundwater.
The patent literature on spent potlining includes a variety of processes for recovery and disposal. King, U.S. Pat. No. 3,151,934 (Oct. 6, 1964), discloses a process involving crushing of the potlinings followed by treatment with a sodium hydroxide solution to extract fluoridic values and dissolve metallic aluminum. Vodic, U.S. Pat. No. 3,606,176 (Sept. 20, 1971), discloses a process involving crushing followed by mechanical screening to remove the metallic aluminum, then further size reduction and the formation of a salt water slurry. The bulk of the carbon fraction is then separated from the cryolite, alumina and residual aluminum by flotation. In each of these processes, however, environmental hazards remain in the disposal of the carbon discards. A process disclosed by Williams, U.S. Pat. No. 3,635,408 (Jan. 18, 1972), involves a dry steam treatment in a rather complex operation.
More recent processes involve pyrolysis as a means of recovering fluoridic values. See for example Bell, et al., U.S. Pat. No. 4,113,832 (Sept. 12, 1978), Andersen, et al., U.S. Pat. No. 4,158,701 (June 19, 1979), Andersen, et al., U.S. Pat. No. 4,160,808 (July 10, 1979), and Andersen, et al., U.S. Pat. No. 4,160,809 (July 10, 1979). Again, these are complex and costly operations. Further processes include treatment with carbon and aluminum fluoride to remove impurities, as disclosed in Bylund, et al., U.S. Pat. No. 3,528,801 (Sept. 15, 1970); oxidation and agglomeration in a fluidized bed, as disclosed in Roberts, et al., U.S. Pat. No. 4,053,375 (Oct. 11, 1977); incineration followed by caustic leaching, as disclosed in Snodgrass, et al., U.S. Pat. No. 4,444,740 (Apr. 24, 1984); and high temperature treatment with silica addition to form slags, as disclosed in Davis, et al., U.S. Pat. No. 4,735,784 (Apr. 5, 1988).
Still further literature on spent potlining and possibilities for its use includes Spironello, V. R., et al., "An Evaluation of Used Aluminum Smelter Potlining as a Substitute for Fluorspar in Cupola Ironmelting," Report of Investigations--United States Department of the Interior, Bureau of Mines; 8350 (1981); Walkup, P. C., "Presentation Synopsis--Use of Molten Salts to Recover Chemical and Energy From Spent Potlining" (1981), both included in Proceedings of the Workshop on Storage, Disposal and Recovery of Spent Potlining, Dec. 3-4, 1981, Aluminum Association, Inc.; Byers, R. L., et al., "Spent Potlining Update," Journal of Metals, November 1986, pp. 34-38; Blayden, L. C., et al., "Spent Potlining Symposium," Journal of Metals, July 1984, pp. 22-32; Williams, M. M., "Recovery of Values From Metal Skim and Potlinings," Paper No. A72-64, pp. 203-224, The Metallurgical Society of AIME (1972); Augood, D. R., et al., "Potlining Flux in Making Steel," included in published reports of meetings of the Light Metal Committee of The Metallurgical Society of AIME, pp. 1091-1103 (1983).
Further information of possible relevance to the present invention may be found in Evans, U.S. Pat. No. 4,337,929 (July 6, 1982); Ormesher, U.S. Pat. No. 4,486,228 (Dec. 4, 1984); Bauer, et al., U.S. Pat. No. 4,501,614 (Feb. 26, 1985); Ormesher, U.S. Pat. No. 4,518,424 (May 21, 1985); and Vire, U.S. Pat. No. 4,568,430 (Feb. 4, 1986). These patents disclose methods and equipment for the use of molten salt in the reclamation of aluminum from scrap.